Forza Horizon 5 runs on the old engine and is an update to Forza Horizon 4 tech. Forza Motorport is built on the new engine which is built with Series S|X in mind.
At 54:40 We experienced our new tyre model, our new tyre pressure model. We have a new way that heat interacts with tyre pressure, we actually have dynamic track temperatures affecting this all. We have dynamic rubbering in on the track. We actually have a new atmospheric pressure system that...
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Yes, but even after the new engine is done, that doesn't necessarily mean the new tech will for sure be impossible to scale down to Xbox One (much less "port" it with a reimagined, pared-down version,
which is always possible.)
There's no engine on the market yet that is impossible on past-gen devices; maybe that will come, but so far, scaling keeps up to a degree. (At least, until new driver sets and APIs come along with just have no business being ported down come out and truly sever the gens.) UE5 is just about the most advanced technology the public has seen, yet it runs just fine when kicked down to Mobile and Switch; you cannot use Lumen or Nanite with those platforms (although
Nanite does work on PS4 and Xbox One, though in an experimental fashion that Epic does not recommend for consumer product projects,) but there are fallbacks and cutbacks which can be made once the project is built at its highest scale. Ratchet & Clank Rift Apart is the most next-gen title we have, with advanced streaming use and graphical techniques inconceivable anywhere near level of quality on previous hardware... yet it uses the same general technology system as the cross-gen Spider-Man Miles Morales.
Maybe a game will come along with an engine that plants a flag in the ground as "Next Gen Only". Maybe it'll go the way it's going, where port-downs are possible but the old hardware isn't as popular anymore (already games are coming out PC and Switch but then PS5 and Xbox Series) and eventually the level of detail and sacrifice isn't worth the effort and compromises. For now, though, cross-gen does not mean that it's a lesser game on the bigger hardware, and it's still common practice because it's economical and because it's doable.